If you see a progress bar when your Mac wakes from sleep

When your Mac wakes from sleep, you might see a progress bar on its display. This is normal behavior that means your Mac is waking from Safe Sleep.

Safe Sleep ensures that data stored in the main memory of your Mac isn't lost when the computer shuts down due to its battery being drained completely.

Before your Mac enters sleep, Safe Sleep automatically saves the contents of its main memory — like desktop settings, open applications, and other work in progress — to the hard drive.

There are two situations when your Mac will enter Safe Sleep:

If your computer's battery becomes completely drained while the computer is asleep, the computer will shut down. When you connect a power adapter, the computer can be restarted. Press the power button and your Mac will automatically return to the desktop state it was in before going to sleep, making it easy for you to continue with your work.

OS X Mountain Lion v10.8.2 supplemental update 2.0 introduced a feature that lets your Mac enter Safe Sleep after four hours of being connected to AC power. This complies with European Energy Standards (ErP Lot6) and happens when there's no network activity and no activity from connected devices like external hard drives.